Electromagnetic radiating or receiving apparatus



- Ma1 -h24, 1953 R. J. LEES ETTAL ELECTROMAGNETIC RADIATING' 0R RECEIVING APPARATUS Filed Aug. 5, 1947 3 Sheets-Sheet l 2m mmnm nun-J w Km T F W 2 w s wa .9 T5 N wnmu 2 44 550 w Z RD pi 5 a 3 .v 3 2 u w 2 March 24, 1953 L s 2,632,851

ELECTROMAGNETIC RADIATING OR RECEIVING APPARATUS Filed Aug. 5, 1947 s Sheet-Sheet 2' .INVENTOR H 6150mm)? C. H. Wzsrcorr ff! .1 L655 ATTORNEY March 24, 1953 R. J. LEES ET AL 2,632,851

ELECTROMAGNETIC RADIATING OR RECEIVING APPARATUS Filed Aug. 5, 1947 3 Sheets-Sheet I5 miean; for

re fa 79/77 a n/erm a 8601/? b/ II/O 7" f, G BINIZENTORS we ic r f Kfl-l- Lees {lif /71 5 Patented Mar. 24, 1953 ELECTROMAGNETIC RADIATING OR RECEIVING APPARATUS Roland J. Lees, Malvern Wells, Henry .G. Booker, Cambridge, and Carl H. Westcott, Birmingham,

England Application August 5, 1947, Serial No. 766,226 In-Great Britain March 23, 1944 Section 1, Public Law 690, August 8, 1946 .Patent expires March 23. 1964 21 Claims.

"ating or receiving aerial of a slot in'a metal sheet.

It has been found that a close connection exists between the behaviour of such a slot and a conducting strip or a rod'of the same length as the slot, the width of the slot being equal to the width of the strip or twice the diameter of the rod. The polar diagramsof sucha slot and strip or rod are the same, but the electric and magnetic fields are interchanged.

According to thepresent invention, a radiator or receiver of high frequency "electro-magnetic energy comprises a sheet of metalor other conducting surface formed with a slot which is longitudinally resonant or substantially resonant at the frequency of said high frequency energy. The metal sheet or its equivalent may be flat orbent to facilitate co-oper'ation of the radiating or receiving slot with a reflector.

According to a further feature of the invention, the metal sheet may be shaped to form a substantially enclosed cavity of, for example,

box form, means being provided to supplyenergy to, or to take-energy from the cavity and the dimensions-of the cavity may be such that it is resonant at thefrequency of the energy to'be radiated or received.

More than one "slot may be formed in the metal sheet, and, for example, two slots arranged vertically one above another are provided in one construction embodying the invention, such an arrangement permittingoverlapping or spaced polar diagrams to be'obtained. Radiation from, or reception by, the slots may be controlled by any convenient switching means, and, for example, such switching may be effected by providing a short circuit across the centre of a slot to suppress radiation from, or reception by, that slot.

In particular forms of apparatus embodying the invention, energy is fed to, or taken from the slot by wires connected to opposite sidesof the slot at its centre. I

In order that the invention may be more clearly understood and readily carried into effect certain forms of apparatus constructed in accordance with the invention will now be de scribed ingreater detail by way of example, with which Figures 1, 2, 3 and '4 show simple forms of such apparatus,

Figure 5 shows a practical-construction of radiator or receiver intended for use with a parabolic reflector,

Figure 6 shows a modified form of the radiator shown in Figure 5 having two vertically arranged slots in'a cylinder-together with switching means for the slots,

Figure 7 illustrates an alternative method "of effecting switching at aslot,

Figure 8 illustrates-the relative position of the apparatus of Figure 5 and an associated reflector.

Referring to Fig. 10f the drawings, a square metal sheet I the sides of which are each one wavelength long'at the frequency of the energy to be radiated has a vertical slot 2 cut at its centre,'the-length of the slot being equal'to about half the said wavelength. A second metal sheet 3 of the same dimensions as the first is placed at say, a quarter of 'a wavelength behind the sheet I. The energy to be radiated when the slot is used as a transmitting aerial, is fed by wires [5 and 16 constituting a balanced transmitting lineand connected to the points '4 and 5 at the centre of the slot. Centre feeding as described, -is not essential but has been found to give maximum band width, and feeding at any other point involves'a diiferent input impedance at the feed point.

The two opposite edges of the slot may beregarded as forming a transmission line which is short-circuited except for the half wavelength constituting the slot. The slot radiates considerably more than --a corresponding twin -wire transmission line because of the extent to which current associated with the slot spreads over the metal. During one half cycle, current flows round the ends-oi the slot in on'e direction and charges-of opposite:signaccumulate on the opposed edges of the slot. During the next half cycle, the current flows round the ends of the slot in the opposite direction and charges of reversed sign accumulate on-the opposed edges.

The energy radiated by the slot described has a polar diagram similarto that of a conventional dipole and reflector of the same dimensions but is horizontally instead oi verticaliy polarised.

In the form shownin Fig. 2 which is adapted for use withadirecting paraboloid the sheet I isbentalong vertical 1ines- 6'l on either side of the slotin the manner shown, and the re- 7 fleeting sheet is similarly bent so that "the combination :is convex towards the paraboloid,

the latter not being shown in the drawing.

In Fig. 3 the fiat sheet I is provided with a hemispherical cavity 8 behind the slot 2, the rear of the sheet being shown. The radius of the cavity 8 is conveniently one quarter of a wavelength at the operating frequency.

Fig. 4 shOWs a combination of the arrangements of Figs. 2 and 3 only a sector of the hemispherical cavity 8 being usable because of the rearward bending of the sheet i. The rearward bending helps to increase the filling of the paraboloid which collects energy radiated sideways from the slot.

Referring now to Fig. 5 of the drawings, a simple practicable embodimentof the invention for radar equipment is shown, in which the radiating or receiving slot is cut in the wall of a cylinder 9, one wavelength high, and having a diameter of say one quarter of a wavelength. The latter dimension is too small to cause waves to propagate along the cylinder in the manner ofa wave guide, and it is not necessary in such a case for the end of the cylinder to be closed in order to prevent the appreciable escape of energy by radiation from the ends.

The cylinder 9 is supported by pairs of fins l0 and ll which are clamped by the nut and bolt fastenings shown to the main feeder tube l2 and hold the cylinder by strengthening bands l3 and It. The supporting fins are maintained 'in a vertical position in order that they shall interfere as little as possible with the horizontally .polarised radiation from the slot 2.

, Feeder wires 15 and i6 constituting in the present case, a 260 ohm balanced open line one quarter wavelength transformer, are connected to two projections at the centre of the slot as shown and pass into the main feeder tube i2 through a polythene moulding. The feeder cable I! passes down the centre of the tube 52, the moulding at the aerial end of the feeder being made continuous with the polythene of the cable. A compensating stub is provided consisting of two concentric lines, each of 47 ohms, and having their outers l8 bonded together and their ends shortcircuited by a strip Hi. This compensating stub will be referred to in greater detail later. The

.cable I! is connected to a moulded socket 20, ,shown for convenience below the main portion of, Fig. 5, and a further length of cable 21 is connected from the socket 25 to a similar socket in a feeder line from the transmitter itself.

In order to tilt the beam radiated from the stub 18 it has been found that when the operating frequency is increased then the admittance of a centre fed half wave slot alters. The conductance increases slowly and the imaginary susceptance increases more rapidly, that is to say the slot becomes capacitative. Similarly, when the frequency is decreased the slot becomes inductive. When this admittance is transferred through the 260 ohm, one quarter wave transformer, the susceptance changes with frequency in the opposite direction. The stub, which is a short circuited one quarter wave at the midband frequency, becomes capacitative at higher frequency and inductive at a lower frequency. If, therefore, such a stub is placed at the end of a 260 ohm transformer its susceptance changes will be in the opposite direction to those of the load. If the impedance of the stub is chosen correctly the best compensation can be obtained. To compensate the large susceptance changes involved a very low impedance short circuited one quarter wave stub would be necessary. The effect of the stub can be magnified three times by making it three-quarters of a wave length long and then the impedance necessary is 95 ohms. This is made up of the two 47-ohm lines I8 effectively connected in series.

In order to give a desired field of radiation two radiators and their associated paraboloids as described with reference to Fig. 5 may be mounted side by side, but it will be understood that a single radiator or more than two radiators may be employed as desired.

Referring now to Fig. 6 of the drawings, a modification of the cylindrical form of radiator described with reference to Fig. 5 is shown, the primary purpose of which is to provide a split beam for height finding in radar technique or for other uses.

A cylinder 22 has a long slot 23 constituting in association with two switching bands, two separately radiating slots, one of which is operative while the other is inoperative. The two effective portions of the slot have separate feed points and within the cylinder two rotatable bands 24 and 25 are arranged so that in the positions shown the upper part of the slot 23 will radiate down or up respectively. For this purpose the 7 tube I2 is supported by an arm A which is pivoted -at pivot P and rotated about the pivot somewhat by motor M.

The band width of a conventional half wave lated by the formula ZeZd=constant, where Zd is the impedance of the fiat sheet dipole of the same shape as the lot. A wide slot is therefore necessary to provide a wide band radiator.

@ The band width of a slot or of a flat dipole is \equal to that of a cylindrical dipole of a diameter half the width of the slot or flat dipole, so

that the band width of a slot 5 cms. wide is the same as that of a dipole 2.5 cms. diameter.

Referring now to the use of the compensating .dipole is roughly inversely proportionl to the .the slot.

as an independent slot, the lower end of which is defined by the upper edge of the band 25. The feed points of the lower portion of the slot are effectively short-circuited by the band 25 and radiation from that portion is suppressed. The band 25 is placed so as to have a high capacity to the inner edges of the slot.

Simultaneous rotation of the bands, through a sufficiently large angle causes the feed points of the upper portion of the slot to be short-circuited and radiation from that portion to be suppressed, while the lower edge of the band 24 defines the upper end of the lower portion of I The band 25 is removed from the lower feed points so that the lower portion of the slot now radiates.

The action produced is due to the fact that in the position of the band 25 as shown, the input impedance at the feed point is changed in such a way that a simple stub system will serve to divert the power almost entirely to the other feed points, this arrangement being effective over a wide frequency range. Further, the band serves as a conductor transverse to the slot through which the radiating currents can flow, thus forming a temporary end to the slot. The reactance due to the capacity at either end of the bridge can be madelow but any remaining effect can be compensated by :a suitable change in the length of the slot.

It will be understood that the arrangement described with reference to Fig. 6 may be extended to include any number of slots which may be switched as. desired, and they may be arranged to provide overlapping diagrams. In the latter case a form of capacity switch fitted inside the radiating system may be employed instead of mechanical short-circuiting due to conductors.

It may be desirable for some purposes to have polar diagrams spaced or overlapping in a horizontal plane and obviously the construction described With reference to Fig. 6 or modifications thereof maybe used horizontally instead of vertically. The slots may be arranged end to end or maybe staggered, depending upon the pattern desired.

An alternative method of effecting switching of the slots is illustrated in Fig. '7 in which instead of the rotary band, a plunger 26 is'arranged to slide within short cylinders 21, 28 mounted at the feed points. The plunger in a further alternative may be designed to have desired capacity to either side of the slot and may be made adjustable with respect to the slot.

It will be seen that the arrangements described present the advantages of satisfactory operation without adjustment over a wide frequency band and that as the switching arrangements are situated in the aerial head the feeder system is rendered as simple as possible. It will further be appreciated that the arrangements described for obtaining very closely adjacent or overlapping polar diagrams have particular advantages compared with conventional dipoles. .If dipoles are used not only is it physically impossible for them tooverlap but the unused dipoles near energised dipoleswould be liable to upset the radiation field. These effects do not occur in operation with slot aerials as described.

, Other effects are obtainable with slot aerials which are not so readily obtainable with dipoles, and, for example, in any beam system requiring a horizontally polarised wave in which it is convenient to use a. parabolic reflector elongated horizontally, a vertical half Wave dipole at the focus of the paraboloid would fill-the reflector fairly uniformly and economically but the radiation would be vertically polarised. A vertical half wave slot atthefocus would, however, give horizontal polarisation.

V The reflecting sheet described may be con-' nected at its edgesv to the edges of the sheet formed with the slot, so forming a cavity in the form of a box for example, totally enclosed except for. the slots.- If desired, the reflecting face of the box may be brought closer to the slots to conserve space. The reactance thus thrown across the slots may be compensated by modifying the lengths of, the slots. Such. an arrangement could, for example, be builtinto the skin of an aircraft to cut down aero dynamical drag while. providing an efficient aerial. Again, a broadside aerial array may be constituted by aJnumber of slots appropriately spaced, for example at half wavelength distances, in a sheet of metal or other conductive surface. If such a broadside array is built into the skin of an aircraft detrimental aero-dynamical eifectsmay be overcome by filling the slots with a dielectric. A plurality of slots-in a sheet asindicatedpro be provided. Further, such a system could be constructed as a box totally enclosed except for the slots in one face.

In the case of a cavityor box formed with one or more radiating or receiving slots,-the dimensions are preferably, althoughnot essential- 1y, such that the cavity is resonant at the operating frequency. Energy may be fed to or taken from the cavity by means of a two-conductor feeder, one of the conductors being con-. nected to the cavity, while the other is connected to a probe or similar element insulated from but projecting into the cavity.

Slot: arrays as described are particularly suitable for operation at centimetre wave lengths and certain embodiments of the invention have particularly advantageous application to radar technique as stated with reference to Figs. 5 and 6 although their general application to radio purposes is evident. V

" Whereverin these Letters Patent we refer to a slot one half wave length long, it is understood that a slot that is any integral multiple of one half wavelength is an equivalent length.

We claim:

1. An antenna comprising a sheet of conductive material having an elongated narrow slot whichis one half wavelength long at operating frequency, a transmission linefor feeding the antenna including two conductors respectively con-=- nected to points on said sheet which are closely adjacent to the opposite sides of said slot, said points being located intermediate of the ends of the longer dimension of the slot and at a quarter wavelength from one 'end, and control means for short circuiting the slot across its width between said points.

2.,An' antenna system comprising a metallic sheet having an elongated slot bounded on all ducesan arraylia'ving-the' appearanceof a gratsides by the sheet and of length considerably greater than one half wavelength at operating frequency, said slot having substantially greater length than its width, a metallic bridge across the width of the slot at a distance of one half wavelength from one end thereof, and means mounting said metallic bridge for movement away from said position of one half wavelength from said one end.

3. An antenna system comprising first and sec-' ond metallic bridging sheets, and mounting means for said bridging sheets including a me'-- tallic sheet having an velongatedslot of greater length than its width and of greater length than one half wavelength atoperating frequency, said mounting means including means for supporting said bridging sheets and menas for moving said bridging sheets to and from positions across the width of said slot with the first bridging sheet one half wavelength from one end of the slot and the second bridging sheet one half Wavelength from the other end of said slot.

4. An 'antenna'system comprising a cylindrical sheet metal element defining an elongated slot of a length at least as long as three quarters of a wavelength at operating frequency, said slot being parallel to the axis of said element, a first pair of feed lines connected to said metallic element closely adjacentopposite edges of said slot respectively and one quarter of a wavelength from the first end of the slot, a second pair of feed lines connected to said metallic element closely adjacent opposite edges of said slot respectively and one quarter of a wavelength from the: second end. of. the slot, anda of cylindrically shaped metallic bands subtending at said axis an angle of less than 360 degrees, the opposing ends of each band defining a gap of greater width than-the width of said slot, said bands being of a diameter slightly different than the diameter of the first-named cylinder and supported thereby for rotary movement, the first band having its edge that is closest to said first end located at one half wavelength therefrom and being of such width as to extend to the position of said second pair of leads to short circuit them when the first band is interposed across the width of said slot, the second band having its edge that is closest to said second end located at one half wavelength therefrom and being of such width as to extend to the position of the first pair of leads to short circuit them whenthe second band is interposed across the width of said slot.

5.- The system of claim 4 including movable means joining the two hands together for synchronous rotary movement, said bands having their respective gaps staggered at different angular positions and with the arc length between corresponding edges of the gaps at least as great as the width of said slot, whereby said movable means is operable to interpose either of said bands across the width of the slot without similarly interposing the other band across the width of the slot.

6. An antenna system comprising a metallic sheet metal cylinder defining an elongated slot parallel to the axis thereof, said slot being of a length considerably greater than one half wave length at operating frequency and bounded by the sheet on all sides, means operable to alternately interpose conductors. across the width of the slot at distance of one half wavelength from different ones of the ends of the slot, a first pair of feeders respectively connected to the cylinder adjacent opposite edgesof the slot at a distance of one quarter wavelength from a first end thereof, and a second pair of feeders respectively connected to the cylinder adjacent opposite edges of the slot at a distance of one quarter wavelength from the second end thereof.

7. An antenna system comprising a metal cylinder having, a slot parallel to the axis thereof, a support for said cylinder including a portion spaced from the cylinder and approaching the cylinder along a line perpendicular to the axis of the cylinder, and a two conductor transmission line passing through said portion and having its conductors respectively connected to two points on said cylinder which are closely adjacent opposite edges of said slot respectively, said support including two fins, the first of which is connected from said portion to a first end of said cylinder and the second of which is ocnnected from said portion to a second end of said cylinder.

8. A directional aerial system comprising a metal cylinder having a longitudinal slot resonant at operating frequency, a reflector in the shape of a paraboloid of revolution and having its concave surface facing said slot, a supporting structure for the cylinder pivoted about a point which is on the back of the reflector and connected to the ends of the cylinder to permit the two movable bridges of conducting material, means which when in a first position supports the first of said bridges across the slot at a distance of one half wavelength from one end of the slot and supports the second bridge removed from the slot and which when in a second position supports the second of said bridges across the slot at a distance of one half wavelength from the other end of the slot and supports the first of the bridges removed from the slot, said lastnamed means being movable from the first to the second of its positions, and a branched two conductor transmission line having its conductors connected to the sheet adjacent opposed edges of the elongated opening and adjacent points at which said opening is bridged.

10. An aerial for use in a split beam radio system comprising ametallic cylinder defining an elongatednarrow slit therein bounded on all sides by the cylinder and of length considerably greater than one-half wave length at operating frequency, said slit being parallel to the axis of the metallic cylinder, first and second rotatable metallic bands which are concentric with the cylinder and contiguous therewith, said bands subtending angles at the axis of said cylinder less than 360 degrees and the opposing ends of the bands being parallel to said axis and defining gaps, means fastened to each of said bands for synchronously moving the bands to different angular positions about said axis, said gaps having different angular positions about said axis, and means for rotating the last-named means about said axis.

11. An antenna system comprising a bowlshaped reflector having a concave reflecting surface, a metallic cylinder mounted in the bowl shaped reflector, said cylinder defining an elongated slit therein bounded on all sides by the cylinder, said slit being located parallel to the axis of the cylinder and being considerably longer than one-half wave length at operating frequency, movable means shiftable to first and second positions respectively and including means operable when it is moved to the first position to short-circuit two points that are respectively adjacent opposing edges of the slit one quarter wave length from a first end of the slit and operable when the movable means is moved to the second position to short circuit two points that are respectively adjacent opposing edges of the slit that are one quarter wave length from the second end of the slit, a first transmission line having two wires connecting to the firstnamed two points, and a second transmission line having two wires connecting to the secondnamed two points, said movable means including means for removing the short circuit across one of the two sets of points when it is moved to a position at which it short circuits the other of the two sets of oints.

12. A radiating system comprising a metallic sheet having an elongated slit bounded on all sides thereof by the sheet and of length considerably longer than one-half wave length at operating frequency, means for applying excitationpotential of said operating frequency across opposite edges of said slit, and means operable to change the radiating pattern of waves leaving said sheet, said last-named means comprising movable means which when moved to a first position shunts current across points of said sheet closely adjacent opposite edges of said slit and located substantially one, fourth wave length from a first end of the slit, said movable means including means to open the circuit between said points when said movable means is moved to a second position.

13. An antenna system comprising a metallic sheet having at least two elongated slots of length greater than an integral multiple of onehalf wave length at operating frequency, said slots having substantially greater length than width, feeder means for each of said slots, and means for interposing a metallic bridge across the centre of each slot to prevent said slot radiating.

14. An antenna system comprising a metallic sheet having an elongated slot bounded on all sides thereof by said sheet and of a length greater than one-half wavelength at operating frequency, said slot having considerably greater length than its width, metallic bridging means, and movable means operable to interpose said metallic bridging means across the width of said slot at a first position one-half wavelength from the first end of the slot, said last-named means including means operable to interpose said bridging means across said slot at a second position one-half wave length from the second end of said slot.

15. An antenna system defined in claim 14 in which said movable means includes means to interpose said bridging means across said first and second positions alternately.

16. An antenna system comprising a metal sheet having an elongated slot bounded on all sides by the sheet and of length considerably greater than the length at which the slot will radiate at a predetermined operating frequency, said slot having substantially greater length than its width, a metallic bridge touching both sides of the slot at a position equal to half a wavelength at said predetermined operating frequency from one end of the slot, and means mounting said bridge for movement away from said position.

17. An antenna system comprising first and second metallic bridging sheets, and mounting means for said bridging sheets including a metallic sheet having an elongated slot of greater length than its width and of greater length than one half wavelength at a predetermined operating frequency, said mounting means including means for supporting said bridging sheets, and means for moving the first bridging sheet across the width of the slot at a first predetermined position and simultaneously moving the second bridging sheet from across the width of the slot at a second predetermined position, said first predetermined position being at a distance of one half wavelength at said predetermined operating frequency from that end of the slot most removed from said first position, and the second predetermined position being at a distance of one half wavelength at said predetermined operating frequency from the end of the slot most removed from said second position.

18. An antenna system comprising a metal sheet having an elongated slot bounded on all sides by the sheet and of considerably greater length than its width, a first pair of feeder wires which are respectively connected to opposite edges of the slot near the first end of the slot, a second pair of feeder wires which are respectively connected to opposite edges of the slot near the second end of the slot, a first switch for shorting the first pair of feeder wires, a second switch for shorting the second pair of feeder wires and means for closing one of said switches and opening the other at substantially the same time.

19. The system defined in claim 18 including a parabolic reflector facing said slot to reflect the energy therefrom.

20. An antenna system comprising a metallic sheet having two elongated slots at substantially different positions therein, two pairs of feeders one pair of which connects to opposite edges of one slot near the center thereof and the other pair of which connects to opposite edges of the other slot near the center thereof, and means for alternately exciting the slots with respect to each other by alternately shorting and unshorting said slots across their widths.

21. An antenna system comprising a slotted sheet acting as a radiating means, reflector means for reflecting the radiations of said sheet, energy feeding means for feeding the energy to be radiated to said sheet, and means for shorting the slot at different positions along the length thereof to thus vary the position of the effective radiating portion of the sheet.

ROLAND J. LEES. HENRY G. BOOKER. CARL H. WESTCO'I'I'.

REFERENCES CITED The following references are of record in the file of this patent:

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